1. Field of the Invention
The present invention concerns a device for generating a signal having a complex form, more particularly a device adapted to produce rapidly rising wave front impulse type signals. In this device, the signal is formed by approximations of linear segments having variable slopes.
2. Description of the Prior Art
In the field of known techniques for producing short rise time pulses and especially in distance-measurement-equipment type (DME) systems for radionavigation aid of aircraft, it is frequently necessary to create pulse type signals having a well-defined form. In the case of DME systems, for example, two specific forms are necessary:
for N-DME or navigation DME, the form of the pulse must be as close as possible to a gaussian shape or a square cosine shape on each of the leading-edge and trailing-edge of the pulse; PA1 for the P-DME (or Precision-DME) the leading-edge of the pulse must be substantially sinusoidal whereas the trailing-edge must follow a square cosine law.
Different circuits for these systems are known in the prior art.
Among these circuits are the analog circuits based on the controlled discharge of an oscillating circuit utilizing LC elements. The drawback of this solution is the lack of precision between the start time of the modulation and the resulting form thereof. In the framework of DME, this becomes apparent by a lack of precision with respect to the distance measurement taken which, it will be recalled, utilizes as reference time the leading-edge of the pulse.
Circuits utilizing digital techniques are also known, such as those which utilize a digital-analog converter supplying a variable output voltage, according to a determined law, in relation with a defined clock period. The pulse thus obtained is in the form of a series of stair steps and gives a contour close to that desired. Smoothing is thus necessary to obtain the final form of the pulse. The main drawback of this solution is due to the technological limitation which appears when the leading-edge of the pulse becomes short.
In this case, for a good approximation of the curve, it is necessary to utilize a large number of steps, i.e. a very rapid clock and high-speed digital-analog converters. By way of example, for steps of 20 ns, the clock must work at 50 MHz, with a converter adapted to follow this frequency.